Microwave ovens are supplied microwave energy from generators such as magnetrons using electron emission structures such as cathodes or directly heated filaments with high voltage applied between filaments and anode structures, the high voltage being supplied by transformers which produce a relatively wide range of output voltages dependent upon the load, such load dependent characteristics permitting relatively constant operation of the magnetron over a substantial range of input voltages. Such power supplies produce substantial output voltage differences between conditions where no current flows in the magnetron due, for example, to insufficient temperature of the electron source or to an open connection and conditions where a magnetron is properly heated and generating a normal microwave output. It has previously been the practice to supply a filament transformer energized immediately upon turning on the microwave oven, with the high voltage supply being supplied through a time delay of several seconds to turn on the high voltage supply after the magnetron filament has been heated to electron emitting temperature. Such time delays are expensive and are a source of failure. In addition, if there is a failure of the magnetron due, for example, to filament burnout, loss or emission or an open connection, the time delay does not sense such failure and activates the high voltage power supply. Also, as the magnetron heater initially heats up with high voltage applied, magnetron moding can occur in which the magnetron operates in a mode other than the fundamental anode resonant mode which results in excess current being drawn and/or overheating of the filament, for example, by excess back bombardment of electrons.